Police cars with radar speed detectors can enforce highway speed limits where they are present, but the frequency of their presence at any one location is limited by considerations of cost. Their deterrent effect elsewhere largely depends on fear of drivers that when a police car is not in sight it may nevertheless be watching from a concealed position. However, citizen band radios are used by enough drivers to reduce this fear by broadcast of where traffic police cars are present and where they are not. Such broadcasts reduce the effectiveness of dummy police cars by the side of the road once a driver sees that no live officer is in the car and broadcasts the news.